I want to add a 6 shot side saddle to my Mossberg UTG 500A. What type of forearm will work with this.?
I'm not sure I totally understand your question...
Any forearm should work with any side saddle. The side saddle is not mounted to the forearm, but to the receiver or butt stock. When mounted correctly, the 6 shot side saddle will not overlap or touch the forearm in any way (see linked picture below for example).
So any forearm that doesn't extend beyond the receiver should work (which is virtually every forearm). Is that really what you were asking?
EDIT- After some more thought, did you mean that you have a UTG forearm on a Mossberg 500a? That makes more sense. The UTG forearm is not compatible with a side saddle mounted on the receiver, but you can still mount one on the butt stock. If you really want a side saddle on the receiver, then you need to replace the UTG forearm with a factory forearm.
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